1. Field of the Invention
The invention pertains to silicon carbide bodies of low porosity.
2. Prior Art
For a long time, silicon carbide, SiC, has been recognized as an oxidation resistant, heat resistant material, and bodies of that material have been utilized industrially within many fields, such as for electrical resistance heating elements and refractory bricks. However, one drawback with such bodies of silicon carbide has been their porosity, ordinarily of the magnitude of about 30 percent by volume, whereby the capability of the bodies to withstand oxidation is far less than would be the case if the bodies had no porosity.
Therefore, a goal has been to provide silicon carbide bodies having a low porosity, and a number of different ways to that end have been proposed.
One prior way comprises, for sintering together the bodies from silicon carbide powder, the technique, known from the ceramic industry, of utilizing a gradation of the silicon carbide. This method involves a choice of that particular particle size distribution which affords the highest possible degree of packing. Utilizing that technique it has been possible to provide bodies having a porosity of slightly less than 30 percent by volume.
Another prior way of reducing the porosity of the silicon carbide bodies comprises sintering very fine grained powder mixtures of silicon carbide under a high pressure and at a very elevated temperature. By that technique almost porefree bodies have been obtained. It is true that such bodies have very fine properties, but the method is extremely expensive in as much as it requires the use of specially made graphite molds which can ordinarily not be used for more than one or two times each. Furthermore, elongated bodies or bodies having a complicated design cannot be manufactured by this method.
A third prior way of ensuring a low porosity comprises the addition, to the silicon carbide powder mixture, of a material containing carbon or which forms carbon on heating thereof, and sintering that powder mixture into an intermediate body. Then these intermediate bodies are treated with silicon or a material containing silicon, so that a new production of silicon carbide takes place within the pores, i.e. by siliconizing of the carbon.
A fourth, and to a certain extent similar, prior way comprises the steps of first producing by sintering, from a silicon carbide powder mixture, an intermediate body in the shape of a silicon carbide skeleton, and then, by immersing or the like, impregnating this skeleton with a material containing carbon or having the property of forming carbon on heating thereof, which carbon is then siliconized. One example of such a material is furfurol. An alternative to the use of silicon carbide powder as a base material would be to start with a suitable carbon powder mixture or a carbon-silicon powder mixture, which is treated with silicon to form an intermediate body in the form of a silicon carbide skeleton.
The just described prior methods have proven to be excessively elaborate. This is partly due to the difficulty in obtaining an even distribution of the porosity of the intermediate bodies and partly because of the problem that ordinarily a small quantity of free silicon has been found remaining in the bodies after the siliconizing step. This free silicon is subjected to an internal oxidation, which gradually breaks down the material, if the material is used in an oxidizing atmosphere at temperatures about the melting point of silicon or higher. Therefore, it has been necessary to remove the superfluous silicon in a finishing step, which step involves unnecessary costs and has been impossible to carry out without a deterioration of the qualities of the material.
Beyond the above mentioned problems the prior art techniques for the manufacture of low porosity silicon carbide bodies suffer from the drawback that they are unsatisfactory with regard to the protection against environmental pollution.